Bluetooth: controller: Handle fragmented AD without chaining PDUs

Adds support for handling fragmented advertising data over HCI when
CONFIG_BT_CTLR_ADV_AUX_PDU_LINK is not enabled

- Added support for appending advertising data to ull_adv_aux_hdr_set_clear
  and ull_adv_aux_pdu_set_clear via ULL_ADV_PDU_HDR_FIELD_AD_DATA_APPEND
- Updated ll_adv_aux_ad_data_set and ll_adv_aux_sr_data_set to
  handle fragmentation ops without CONFIG_BT_CTLR_ADV_AUX_PDU_LINK

Signed-off-by: Troels Nilsson <trnn@demant.com>